I know how to add properties to a web part and use these value inside the web part.
Can someone tell me how I can add a dependency so that I can only edit a property e.g. when another is checked?
Thanks in advance.
You need to build a custom editor part. Inside this part you can add the code required to change the properties of data entry controls (e.g. enable or disable) and to do data validation while the user is editing the web part property values.
See this blog post by Wictor Wilen for more information.